To use stealth kill mechanics you need to have an AC_Stealth in your character. In this component you can change the data table with stealth animations.
In the DT_Stealth data table you can create a new row if you want to use more stealth kill animations. You should select the animation of the killer (your character) and the killed enemy.
In the Killed animation montage you can set a PlayMontageNotify to play a neck crack sound.